This invention relates to a method and apparatus for continuously sterilizing substances contained in flat and flexible packages at a sterilizing temperature of higher than 100.degree. C. and under a pressure higher than atmospheric pressure, said packages being packed with such a substance as a foodstuff to be sterilized and then sealed, and the innermost layers or portions of the flexible packages being made of heat sealable film.
Various types of continuous sterilization apparatuses have been used for sterilizing cans but batch-type of sterilization has mainly been used in the past for sterilizing flexible packages. The continuous sterilizers utilized to sterilize cans seem to be particularly suitable for mass production and suitable for a larger output of the products of the same type as in the manufacture of canned foodstuff. The total output of flexible packages such as retortable pouches has relatively increased, but it has been rarely the case that a large number of such packages are treated in one factory, so that the demand for continuous sterilizers has been, until recently, small.
However, with the recent tendency toward mass production of retortable pouches in one factory continuous sterilizers have been increasingly in demand.
Another reason for adopting continuous sterilizers is based on the consideration of saving energy and labor. With the batch-type method of sterilization, since successive sterilization cycles are performed independently, the amounts of the heating medium such as steam, and cooling water required for the sterilization process seem to be larger than those for a continuous method. Furthermore, in the batch-type method, considerable labor is required to load and unload the articles to be sterilized. Accordingly, development of an efficient apparatus capable of continuously sterilizing retortable pouches has long been desired.
A third reason for adopting continuous sterilization is based on considerations of food engineering. Usually, cans have been sterilized at a temperature of about 120.degree. C., but it has been proved not only theoretically but also experimentally that some foodstuffs maintain better quality when sterilized in a shorter time at a higher temperature than at a lower temperature for a longer time. Various factors that affect the quality of foodstuffs, such as flavour, fragrance, color and nutritive value. (Vitamins) are chemically unstable and the rate of destruction of these factors caused by being subjected to heat increases 2 to 3 times for each 10.degree. C. rise of temperature as does a chemical reaction. On the other hand, the normal rate of destruction of bacteria (saprogenuous bacilluses) usually increases 10 to 20 times for each 10.degree. C. rise of temperature. From these facts, it can be seen that when food is sterilized at a high temperature the bacteria like saprogenuous bacilluses will be destroyed in a short time but considerable amounts of vitamins will survive.
With regard to heat penetration, since cans are generally cylindrical, it requires a relatively long time before the center of the can reaches the pre-determined temperature during which local burning tends to occur. For this reason, the can must be kept in violent agitation particularly during sterilization. However, a flexible package is flat and its thickness is not so large. These characterics result in a rapid heat transfer, so that the temperature differential between the central and peripheral portions is smaller than in a can. Accordingly, flat and flexible packages are suitable for high-temperature short-time sterilization.
In the case of high-temperature short-time sterilization, the temperature of the contents needs to be rapidly increased so sterilization of canned food requires severe agitation for the purpose of averaging the temperatures at the central and peripheral portions since the temperature differential therebetween is large as pointed out above. However, in the case of such flexible packages as retortable pouches the packages would be damaged when a severe agitation is used because they do not have sufficient strength as compared with cans and in certain cases the sterilized packages may be damaged and there is a risk of spoilage. However, as above described, flexible packages are flat and the temperature differential between the central and peripheral portions is small, so that the flexible packages can be sterilized at a high temperature without applying agitation such as is used in the case of cans.
In high-temperature short-time sterilization, because a high rate of heat penetration into a food contained in the pouch is needed, it is desirable that the temperature of the sterilizing atmosphere should reach a predetermined high temperature as fast as possible (come-up time), so that with a conventional batch-type method requiring a considerably long come-up time, the high-temperature short-time sterilization can not be achieved. For this reason, it is desirable to provide a sterilizing apparatus capable of reducing the come-up and down time close to zero, that is a continuously sterilizing apparatus.
Among various types of continuous sterilizers which can be used for sterilizing cans, those suitable for the sterilization of flexible packages are a hydrostatic-type sterilization apparatus and a sterilization apparatus utilizing a special valve. In the former type, in order to maintain a predetermined pressure in the sterilizing chamber there is provided a water column communicating with the sterilizing chamber at the bottom so as to cause the hydrostatic pressure head in the column to balance with the pressure in the sterilizing chamber. Accordingly, a high water column must be used in order to maintain a high pressure in the sterilizing chamber, thus requiring the installing of a tall column. When steam is used as the heating medium, the pressure is about 1 kg/cm.sup.2 at a temperature of 120.degree. C., about 2.2 kg/cm.sup.2 at 135.degree. C. and about 3.9 kg/cm.sup.2 at 150.degree. C., so that a hydrostatic-type sterilization apparatus is required to use a water column having a height of about 10 m for 120.degree. C., about 22 m for 135.degree. C. and 39 m for 150.degree. C. In this manner, high-temperature short-time sterilization using a hydrostatic-type sterilizer is actually limited by the construction of the apparatus. At present, the maximum sterilization temperature available in a hydrostatic-type sterilizer is said to be about 128.degree. C.
In sterilization apparatus utilizing a special valve as a sealing means, since a chain conveyor for conveying such containers as cans is passed through the valve, it is impossible to prevent leakage of cooling water through the small gap through which the chain conveyor passes. Accordingly, pumping means are provided for replacing the leaked cooling water, and in certain cases a pump having a substantial capacity is needed. The highest sterilization temperature of this type of apparatus is said to be about 143.degree. C.
As described above, these continuous sterilizers are designed for mass production, but in some countries where a large number of pouches are not heat-processed in one factory, it is desirable to provide inexpensive continuous sterilizers having a relatively small size and capacity.
However, in order to save energy and labor a fully automated continuous sterilization system must be used in which not only the sterilization apparatus per se but also various equipment installed before and after can be suitably connected to operate automatically. Otherwise, the advantage of a fully automatic line can not be realized. As compared with such rigid containers as cans and bottles, the flexible packages are flexible and have lower mechanical strength so that there are such problems as wrinkling, scratching, folding, warping and welding seam damage. Thus, there are many problems involved in establishment of a fully automatic processing system.